Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with foil, leaving an overhang on all sides for easy removal. Spray the foil-lined pan lightly with nonstick cooking spray.

In a medium bowl, whisk together 1½ cups (187.5 g) all-purpose flour, 1 teaspoon baking powder, and ½ teaspoon salt until evenly combined.

Break the 14-ounce (396.89 g) Symphony Bars into small pieces and set them aside.

In a large bowl, whisk the 12 tablespoons melted and cooled unsalted butter and 1½ cups (330 g) light brown sugar until combined.

Add the 2 lightly beaten eggs and 4 teaspoons vanilla extract to the butter–sugar mixture and whisk until smooth.

Using a rubber spatula, fold the dry ingredients into the wet ingredients just until no streaks of flour remain. Do not overmix—the batter will be thin.

Pour the batter into the prepared pan and spread it into an even, thin layer with the rubber spatula.

Bake on the middle rack until the top is shiny and light golden brown, 22 to 25 minutes. Do not overbake.

Remove the pan from the oven. Immediately sprinkle the reserved Symphony bar pieces evenly over the hot surface and let sit for 1 minute.

Using an offset spatula (or regular spatula if you don’t have one), gently spread the melting chocolate pieces into an even layer over the bars; the chocolate will continue to melt as you work.

Transfer the pan to a wire rack and cool to room temperature. If the chocolate is still soft when cool, refrigerate for 15 to 30 minutes to firm it.

Use the foil overhang to lift the bars from the pan and place on a cutting board. Cut into squares.

Store the bars in an airtight container for up to 5 days. If your home is warm, store them in the refrigerator.